A Gram-stain-negative, non-flagellated, rod-shaped bacterial strain able to move by gliding, designated WS-MY9T, was isolated from a brown algae reservoir in South Korea. Strain WSMY9T grew optimally at 25 °C, at pH 7.0-8.0 and in the presence of 2% (w/v) NaCl. A neighbour-joining phylogenetic tree based on 16S rRNA gene sequences showed that strain WS-MY9T clustered with the type strain of Algibacter lectus with a bootstrap resampling value of 100 %. Strain WS-MY9T exhibited 16S rRNA gene sequence similarity values of 98.5 and 96.7% to the type strains of A. lectus and Algibacter mikhailovii, respectively, and less than 96.1% sequence similarity to other members of the family Flavobacteriaceae. Strain WS-MY9T contained MK-6 as the predominant menaquinone and anteiso-C15: 0, iso-C17: 0 3-OH, iso-C15: 1 G and iso-C15: 0 as the major fatty acids. The major polar lipids of strain WS-MY9T were phosphatidylethanolamine and two unidentified lipids. The DNA G+C content of strain WSMY9T was 35.0 mol% and its DNA-DNA relatedness value with A. lectus KCTC 12103T was 15 %. The phylogenetic and genetic distinctiveness and differential phenotypic properties revealed that strain WS-MY9T is separate from the two recognized species of the genus Algibacter. On the basis of the data presented, strain WS-MY9T represents a novel species of the genus Algibacter, for which the name Algibacter undariae sp. nov. is proposed. The type strain is WS-MY9T (=KCTC 32259T=CCUG 63684T).